Conventionally, there is known an idling stop system that automatically stops an engine when a vehicle such as an automobile temporarily stops at an intersection or the like.
When an engine speed gradually decreases by an implementation of the idling stop system and immediately before the engine stops, a piston may be pushed back without being able to go over a compression top dead center, which causes a crankshaft to rotate in reverse direction.
If a start request of the engine is generated before the engine completely stops and a starter is operated while the crankshaft is rotating in the reverse direction, an excessive load may be applied to a pinion that meshes with a ring gear of the engine, and thus the pinion may be damaged.
In contrast, a technology that prohibits an act of intentionally rotating a crankshaft (hereafter, this act is called cranking) while reverse rotation of the crankshaft is detected or estimated when a start request is generated in an automatic stop of an engine by an idling stop system has been disclosed in Japanese Patent No. 4228882.
However, in Japanese Patent No, 4228882, until a possibility that reverse rotation does not occur after an engine rotational speed falls below a predetermined rotational speed disappears, in other words, a predetermined time period from when before the crankshaft starts to rotate reversely to when it completely stops rotating, is set as a cranking prohibition period.
Therefore, for example, when the crankshaft repeats reverse rotation and forward rotation, it is impossible to start cranking while the crankshaft is not rotating reversely, that is, while the crankshaft is rotating forward.
As a result, the delay from generation of the start request until the cranking is started increases, causing a problem that the start of the engine is delayed.